1. Field of the Invention
This invention relates, in general, to electrical bushings and, more specifically, to electrical bushings having condenser structures.
2. Description of the Prior Art
Recent developments in power transmission have resulted in systems employing voltages in excess of 765 KV. Accordingly, the power transmission equipment, such as transformers, must increase in capacity and size to handle the extra high voltages. In addition, condenser type electrical bushings normally associated with power transformers must also increase in size and, more particularly, in length. However, the effective use of condenser structures in bushings designed for extremely high voltages presents several problems relating to design and manufacturing techniques uncommon with lower voltage bushings.
In condenser-type electrical bushings, a plurality of electrically conducting layers surround the conductor to distribute the voltage equally across the bushing. However, as the bushing increases in size, it becomes increasingly difficult to provide uniform capacitance between adjacent layers.
Split-foil or axially-divided capacitive layers have been utilized in U.S. Pat. Nos. 3,462,545, 3,627,906, 3,659,033 and 3,793,477, all assigned to the assignee of the present application, to provide uniform capacitance and minimize radial build of the condenser sections. However, this method of voltage control has a length limitation due to increasing voltage gradients with length. There are also manufacturing restrictions on condenser size due to factors such as the width of the insulating paper, and the diameters and lengths of winding machinery. In particular, the restrictions on the maximum available width of insulating paper used to form the condenser necessitates the use of a time consuming spiral winding process to wrap the paper around the conductor when the maximum paper width is exceeded.
Another method of providing voltage control that overcomes many of the problems associated with the split-foil type condenser structures utilizes a so-called modular condenser; that is, a condenser constructed of several smaller sections or modules which are manufactured individually and assembled together to form the complete condenser assembly. For the modular condenser to be effective, however, the voltage gradients across the gap between adjacent modules must be controlled. One method of controlling voltage gradients across the gap is shown in U.S. Pat. No. 3,588,319, issued to T. Isogai et al., in which the gap between opposing modules is disposed at a predetermined angle with respect to the axis of the conductor. In addition, the conductive layers in the condenser module whose face forms an obtuse angle with respect to the axis of the conductor are placed nearer the conductor than the conductive layers in the opposing module so that the equipotential lines between the conductive layers in the two modules intersect the gap at right angles to thereby increase the creepage distance along the gap. However, the different radial spacing of the conductive layers in the adjoining modules may result in regions of high voltage gradients in the condenser.
Thus, it is desirable to provide a condenser bushing in which the individual condenser modules are effectively arranged to provide uniform capacitance across the bushing without producing regions of high voltage gradient concentration. It is also desirable to provide a condenser bushing having individual condenser modules in which a high degree of voltage control is maintained across the gap between adjacent modules. It is also desirable to provide a condenser bushing for use in high voltage applications which can be manufactured by existing equipment and, further, which can use the more economical sheet winding process to wrap the paper insulation around the conductor instead of the time consuming spiral winding method.